This is an instrument designed to measure magnetic declination: i.e. the angle that, at a specific point on the earth’s surface, the magnetic meridian plane - the plane passing through that point and containing the magnetic needle in its position of natural equilibrium - makes with the geographical meridian plane. Magnetic declination undergoes both periodic and regular variations over time and these instruments serve to record such variations.
This declination compass (or dip circle) is "fitted with a compass and graduated circle with movable sights mounted on a black rectangular wooden case, which can be positioned horizontally by means of three metal screws", reads the 1843 inventory drawn up by Gaetano Ceschi. In the Notizie dell’Origine e Progressi dell’Instituto delle Scienze di Bologna ... of 1780, it is reported as "The Declination compass, an instrument described by Quadri in his gnomonic tables, used to determine exactly the declination of the magnetized needle."
Notizie dell'Origine e Progressi dell'Instituto delle Scienze di Bologna... (1780) p. 187.
G.L. Quadri (1733).